There is a small, round, black "organ" under chicken thighs, and some people like to eat it. Identifying this seems to be quite the lingering Internet mystery.

They are there on prepared fried chicken--specifically Popeye's, but I'm sure it's there on any prepared bone-in chicken thigh. I've read various claims that it's the "oyster", liver, kidney, or a blood vessel. By "under" the thigh, I mean that it's found on the bony side opposite the meat.

When I cut up whole chickens myself, the thighs have only one bone, the leg bone (femur). Digging through the links you gave, I eventually found this, which has the ring of truth to me, especially the note on bad cutting: "IF your chicken is so badly cut that the thighs include part of the pelvis, then those lumpy things in the hollow underside are the kidneys. I am always careful to detach the thigh-bone from its socket on the pelvis, and to treat the back (including said pelvis) as a separate piece, to be cooked and then eaten by whoever gets it first. INCLUDING those kidneys!"
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SAJ14SAJJan 28 '13 at 22:27

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This thing looks indeed like a kidney from what details I can see (google "kidney cross section" to see diagrams, be aware that anatomic diagrams mostly show cuts on an axis perpendicular to the one used here). You can also try to chew them - red meat has muscle fibers, even though they are less pronounced in chicken. If it feels sponge-like, it is a kidney (or other innards), not a muscle.
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rumtscho♦Jan 29 '13 at 0:04

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it's definitely a kidney, you can see the vein down the middle in the cross-section picture and the color of it after being cooked is a pretty big giveaway as well. That's really strange though, i've never seen that before in any commercially made fried chicken, even that cut of meat seems to be strange for commercial fried chicken. Where do you live?
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BrendanJan 29 '13 at 3:23

FYI, Popeyes is a fast food chain in the US serving New Orleans style fried chicken, fairly common in the south and southeast, at least. One of their marketing claims is that their chicken is never frozen. They may have their own processing infrastructure, I don't know. Their thighs are consistently cut with two bones in a T-shape, not just the femur as when done by hand. Still, their chicken is delicious, the only fast food that I actually like.
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SAJ14SAJJan 29 '13 at 7:20

I've had popeye's plenty of times, I don't recall them ever being cut this way though, I wonder if it's a regional thing.
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BrendanJan 29 '13 at 14:59

10 Answers
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As discussed in the comments under the question, I believe the organ in question is a kidney, from the pocket in the pelvis of a thigh butchered in a fairly unusual manner, with that part of the pelvis still attached.

See page 3.21 in the University of Kentucky's PDF of Chapter 3 of Chicken Anatomy and Physiology. It shows where the kidney's are in the chicken (moderately graphic), and the shape looks quite similar to the mystery item in the original question's photographs, allowing for shrinkage from cooking.

There are no organs on the underside of a chicken, the oyster is simply a bite-size piece of muscle which is tender and usually the tastiest piece of the whole bird. Two of links are about the oyster, which isn't gizzard-like at all. The first link is asking what the livery tasting stuff that sometimes comes attached to chicken thighs is, which is in fact liver left by poorly executed preparation by the packaging company.

I added photos for clarification. Partially-eaten Popeye's chicken for reference. I don't see how this could be poorly executed preparation; I'm pretty sure this "gizzard" is in all chicken thighs.
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Jeff AxelrodJan 28 '13 at 20:16

From the looks of the photos what you're seeing is part of the "oyster". It can sometimes be darker in colour. It's the muscle found near the base of the back where the thigh meets the body.

The texture is slightly different than other parts of the bird. When cooking a whole bird you pop that bit of meat it's concave bone area and it resembles an oyster in shape.

I provided a link to a webpage that has some nice pics of a bird being cut up. You'll see about 3/4 down tha page they have the body split down the breast bone showing the inside along the back bone without legs attached. Notice no organs attached anywhere. I've NEVER seen any organs still attached inside the body of a comercially cleaned chicken and I've seen more than a truck load of birds in my 15+ years of cooking professionally.

Here's another good pic for you. It's the concave bone of the pelvis that holds the oyster. When they cut the bird up for fried chicken they cut the bird in two, down the spine then make three cuts per side and seperate the dumb stick. One half way up the pelvis, the next one splits the upper body into a wing section and a breat section. This way you'll end up with 4 fairly equal sized pieces that should cook at about the same time.

From the newest pictures in the question, I cannot agree on the conclusion that the part being asked about is the oyster, which is normal muscle tissue; the item shown in the detail clearly looks like a non-muscle organ of some type. Popeye's may not prepare their chickens in a standard commercial manner in accordance with your experience.
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SAJ14SAJFeb 2 '13 at 0:08

I am not sure what the organ inside the chicken pelvis is, BUT it reminds me of, what in beef, is called the "sweet bread". They have one thing in common, they both taste like the liver of that animal, but with a lighter, more delicate flavor. I have also eaten pork and beef kidneys -- what is called the "gizzard" is closer in both texture and flavor to a kidney.

Those are the kidneys. I just finished butchering eight chickens yesterday. They look exactly like a little kidney bean (lol). I don't believe they will hurt you, after all people eat beef kidneys, kidney pie, etc. kidneys are usually removed along with everything else. They can be easily popped out with your finger.

What first must be said, I love chicken cooked in any way you can think of!

But I always remove what is left of the kidney from my chicken thighs, and any other organ left before cooking. I have eaten this before, and I care nothing for it's strong flavor. And I have learned from experience: cooking thighs, and even a whole chicken with any of these parts of the kidney still in it will give all the meat, even the breast, a hint of it's strong flavor.
If you enjoy this flavor then by all means leave it in through the cooking process!

Here in Sydney Australia it's always there, always. Fresh chook(whole or thighs), charcoal chicken(bbq), KFC(Kentucky fried chicken), whatever. It's definitely not the oyster. I thought they might be testicles, but they would only appear in roosters, not chickens. I also thought they might be a kind of bone marrow deposit/reservoir like what humans have in our hips(the putty like stuff they do bone grafts with when you badly break a bone). IMHO, whatever it is, it's the best bit of the bird there is, YUM!!!